Microbiological changes along a modular wastewater reuse treatment process with a special focus on bacterial regrowth

Abstract

Water reuse is becoming an increasing necessity due to depleted water resources or increased water demand. A treatment process on a pilot scale was designed to produce different water qualities for different applications in industry or agriculture. We report here microbiological changes along the modular process using treated municipal wastewater effluent as raw water. Treatment technologies included coagulation, ultrafiltration (UF), reverse osmosis (RO), quartz sand, activated granular activated carbon (GAC) filtration and disinfection. Elimination of traditional hygiene indicator bacteria was already achieved by ultrafiltration as the first barrier. Profound changes by each treatment step also applied to the microbiome. Total and intact cell concentrations as quantified by flow cytometry underwent a strong decline after UF and RO, whereas biological stabilization was achieved through quartz sand filtration and GAC passage. Interestingly assimilable organic carbon (AOC) was still present even after RO at levels that allowed substantial regrowth of bacteria. Overall, UF and RO led only to a 0.43 and 0.78 log decrease in intact cells concentrations in stagnated water after regrowth compared with 6.5 log intact cells/ml in the stagnated raw water. Temperature was shown to be an important parameter determining the microbiome of the regrown population. Regrowth could, however, be efficiently suppressed by monochloramine. HIGHLIGHTS A versatile water reuse process was established producing different water qualities (MULTI-ReUse).; Elimination of traditional hygiene indicator bacteria was already achieved by ultrafiltration as the first barrier. This predestined ultrafiltration as a critical control point for high-resolution process monitoring by online flow cytometry that produced data on total and intact cells in high temporal resolution.; Offline cytometry allowed the assessment of the change in regrowth potential along the treatment process.; Despite strong reductions in bacterial concentrations by UF and RO, the suppression of bacterial numbers was not sustainable as the regrowth potential (as determined by offline flow cytometry) was only reduced moderately (on a log scale) despite a strong reduction of DOC (on a linear scale).; Flow cytometry proved a very sensitive diagnostic tool both in an offline and online format.;

Keywords

• bacterial regrowth
• flow cytometry
• process monitoring
• water recycling
• water reuse